This is a very important question. Before giving a talk, try to figure out the projector resolution.
(I think most of them are 1024x768.) I once got the projector replaced just before my talk,
so I found myself trying to edit the .css files right in front of the audience.
(Nice exercise!)

Linux has a command called xte, which can position the mouse cursor at a given point
on the screen. For example:

xte 'mousemove 1024 768'

Maybe this can be used to check if your slide will fit.

The easiest way to adjust the width of the slide is by using the width
parameter of .maincolumn.

Alternatively, you can change the font sizes. If you do decide to change the font sizes, you
might also need to adjust the formula size in the headers of the .scrbl file, and
perhaps in the @fsize[] function.